CHICAGO (CBS/AP) — An investigation shows that the Chicago Police Department has failed to closely monitor the way it handles officer overtime.

In a report released Tuesday, the office of Inspector General Joseph Ferguson says that the lack of oversight has left the department vulnerable to “potentially abusive practices” as they accrue overtime.

The office says the department — which relies heavily on overtime in recent years to stem the surging gun violence in the city — has paid out millions of dollars to officers whose overtime was not properly approved or to officers who even approved their own overtime payments.

The department has spent more than a half-billion dollars in the last six years on overtime and the report also raises concerns that the overtime has left officers fatigued.

Fraternal Order of Police Lodge 7 was critical of the audit, saying it doesn’t take into account that officers make arrests late in their shift.

“One way to lower the overtime rate is to finally hire more police officers, as the FOP has been asking for years and has been promised time and again,” a statement from the union said.

Chicago Police Supt. Eddie Johnson was on hand Tuesday to address the audit, saying the department is testing a new electronic system to track overtime.

He recently underwent a kidney transplant and was on medical leave while recovering.

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